1. Field of the Invention
The invention relates to a method for detecting combustion misfires in a multi-cylinder internal combustion engine by evaluating the rotational speed of the crankshaft, which includes measuring segment times required by a crankshaft to pass through prescribed angular ranges during working cycles of individual cylinders; correcting the segment times by a correction factor including mechanical tolerances of a rotational speed pickup; calculating unsmooth-running values from the corrected segment times; and comparing the unsmooth-running values with a threshold value and registering a combustion misfire if the threshold value is exceeded.
The occurrence of combustion misfires in an internal combustion engine can lead, on one hand, to an increase in a rate of emission of pollutants and, on the other hand, to destruction of a catalytic converter disposed in an exhaust-gas tract of the internal combustion engine, or at least to degradation of its conversion capability due to post-reactions of an uncombusted air/fuel mixture.
The detection of such combustion misfires is therefore required in order to monitor compliance with legal limits for the emissions during operation. Detecting the combustion misfires from the crankshaft speed measured through the use of incremental sensors provides a cost-effective way of implementing that objective.
Therefore, a plurality of methods have already been disclosed which detect combustion misfires by measuring segment times that the crankshaft requires during working cycles of individual cylinders in order to pass through prescribed angular ranges. Then, unsmooth-running values are calculated from the segment times and those values are compared with threshold values. Errors in the segment time measurement are detected and corrected during times of overrun fuel cut-off conditions of the internal combustion engine. Such a method is disclosed, for example, in European Patent Application 0 583 496 A1, corresponding to U.S. Pat. No. 5,377,536.
Combustion misfires lead to a temporary slowing of the angular speed of the crankshaft. That effect is very small so that the angular speed has to be determined very precisely.
Therefore, European Patent Application 0 576 705 A1 proposes that the general rotational speed tendency and additionally inconstant changes of rotational speed should be taken into account during the measurement of the rotational speed in a method for detecting combustion misfires, in addition to a statistical component, so that incorrect detections can be very largely excluded even when operation is very far from steady-state conditions.
Tolerances and reproduction deviations relating to manufacturing (for example mechanical gear faults) or relating to the installation of the incremental sensor on the crankshaft (for example eccentric mounting) also lead to inaccuracies when measuring the angular speed and thus to possible incorrect detections when detecting combustion misfires.
European Patent Application 0 583 495 A1 discloses a method for detecting and correcting errors when performing time measurements on rotating shafts, in particular on crankshafts. In that case, measurements are made of segment times which the shaft requires in order to rotate through a defined angular range (segment), and those times are then compared with a time which applies for a reference segment. When the internal combustion engine is in an overrun mode, a correction value is determined which permits the measured segment time to be corrected on either a cylinder-specific or segment-specific basis.
Furthermore, reactions of the road and the mechanical behavior of the crankshaft itself influence the speed behavior of the crankshaft and thus make it more difficult to detect combustion misfires. In particular, in the case of multi-cylinder series-mounted engines with a long crankshaft, torsional vibrations occur which can no longer be ignored and which may falsify the result of a detection of combustion misfires. That can take place to such an extent that in higher rotational speed ranges it is no longer possible to ensure that the detection will be reliable for all of the cylinders when using conventional methods.